1. Field of the Invention
The invention relates to a method and system for coating a traveling material web, such as a paper web or cardboard web.
2. Description of the Related Art
Coating systems for paper and cardboard are variously known where, for instance according to GB 21 03 115 A, the transfer of coating mixture on the material web, from the shells of two coordinated rolls forming together a press gap, takes place in the press gap. Employed as an applicator system are nozzle applicators or dip rolls. For smoothing and dosing of the applied layer, the applicator system uses a doctor blade.
Known for the coating of traveling webs are also applicator systems which operate in the fashion of a curtain coater and where the uniformity and adhesion of the curtain coat is improved in that the system is preceded by a vacuum chamber arranged on the backing roll that carries the web. A curtain coating method employing a vacuum chamber is described in U.S. Pat. No. 3,735,729, which is not customarily used for paper webs. It serves primarily the coating of photographic paper, since the respective coating mixtures possess a high consistency. Considerable problems are encountered in the coating of paper webs with sizing, which have a very low consistency and viscosity.
All of these inventions are suited for the application of relatively large quantities of coating mixture per square meter (m.sup.2) of web area. But they fail when an extremely low coating weight per m.sup.2 is desired.
Known from EP 04 35 904 B1 is a method and a device for the coating of paper, cardboard or similar substrates that travel continuously along a path. The coating is transformed to a mist and fed to an applicator nozzle with an applicator surface. A vacuum chamber for pickup of surplus mist is arranged adjacent to the applicator surface. The spray mist created is by a "positive force" (pressure) and by means of the applicator nozzle applied directly on the substrate (paper web). With this method, too, it is doubtful whether an extremely low coating weight per m.sup.2 can be applied on the paper web with the uniformity that is necessary for many paper grades.
Lastly, with so-called air knife systems it is also known to arrange the air knife setup in a large vacuum container. Here, however, the air knife does not apply a coating, but serves to scrape coating mixture off the web in order to dose it.
The problem underlying the invention is to provide an applicator system, or coating system, for traveling material webs involving very light, extremely thin coatings and nonetheless uniform amounts of coating, notably for the application of sizing.